High-temperature and pressure seismic properties of a lower crustal prograde shear zone from the Kohistan Arc, Pakistan
Résumé
Anastomosing shear zones exposed in the lower crust of the Kohistan palaeoisland arc in Pakistan have parageneses indicating increasing pressure during deformation. Therefore, they represent a rare example of strain localization during crustal thickening. We investigated the seismic properties of a sheared gabbro with constant bulk chemistry across one of these shear zones. The compressional wave velocity was measured at confining pressures of up to 0.5 GPa and temperatures of up to 600 8C. The density, the average Vp (compressional P-wave velocity) and the acoustic impedance at room temperature increase from the undeformed protolith through the strain gradient to the intensely sheared mylonite. The seismic anisotropy is largest for the strain gradient. The Vp dependence of the velocities on temperature is much higher in the mylonite than in the protolith. The acoustic impedance contrast between protolith and mylonite is high enough to generate seismic reflections at low temperatures, but not at high temperatures. This suggests that the seismic reflectivity may also depend on the temperature. Consequently, ductile shear zones can be detected in reflection profiles of crusts with low geothermal gradients, and may be transparent in crusts with high geothermal gradients.